This invention relates to methods of making optical waveguides and optical waveguide preforms and products, and more particularly to providing such products by vapor deposition techniques.
The most commonly used techniques for optical waveguide manufacture at present are based on dissociation in a flame of glass forming constituents to build up a porous preform of vitreous particles called "soot". The "soot" preform is converted to a glassy state by sintering at an elevated temperature. A desired combination of core and cladding layers is drawn under temperature and tension to its desired final diameter. As the usage of optical waveguides has increased, the technical requirements for communications systems have also become more demanding. In the current state of the art signal attenuation of 0.4 dB per kilometer, low dispersion, and precise cut-off wavelength characteristics are often sought. These factors dictate that, in addition to very low impurities and a homogeneous microbubble-free structure, there must also be reliable and predictable control of the refractive index profile as defined by the core and cladding geometries and materials. In addition, of course, since the economics of any process used are of great importance, basic costs and yields must both be at satisfactory levels.
In consequence of such factors, processes heretofore used are now less than satisfactory from one or more standpoints. An early widely used process was so-called "inside vapor deposition," also known as the modified chemical deposition process, in which a core soot was deposited on the interior of a specially prepared silica tube, and the body was after vitrification collapsed down to fill the interior prior to or during drawing. The hollow silica tube is costly to prepare and restricts the preform size that can be made using this process. Other widely used alternatives now include an outside vapor deposition process, which is a radial deposition technique. The outside deposition process requires deposition on a mandrel and subsequent removal of the mandrel prior to sintering. This operation is sensitive and imposes a length restriction on the preform. In about 1977 a vapor-phase axial deposition process was devised for continuous soot deposition and preform manufacture. This method is described in Vol. 1, page 97 et seq., of the book Optical Fiber Communications, entitled "Fiber Fabrication," edited by Tingye Li, and published (1985) by the Academic Press, Inc. It is characterized by directing a soot stream toward the central vertical axis of a rotating target and providing relative axial movement between the stream and target as a solid cylinder of material is deposited. This solid cylinder in the VAD process can be continuously sintered, if desired, to provide a glass start rod, or cladding soot can be concurrently deposited by synchronously traversing the outer diameter of the cylinder with a soot stream impacting in a radial direction. The theoretical process advantages derivable from this approach are inhibited by a number of practical factors and technical limitations. The upwardly directed axial soot stream must be diverted away from the sides of the deposited cylinder to prevent side deposition, and for this purpose secondary air flows and special exhaust configurations must be used, along with precise burner control. Moreover, when radial deposition is to be used concurrently, it is difficult to synchronize the radial buildup rate with the axial buildup rate, which is not uniform due to instabilities in the chamber. Also, with both burners in the same chamber there is inevitable mixing of the soot streams and a diffuse interface between the core and cladding.
There are, in addition, certain fundamental limitations on the VAD process as now practiced. As described in the referenced article, and in U.S. Pat. No. 4,224,046, the initial thinking was that the soot stream should be coaxial (=0.degree.) as well as vertical. Subsequent workers proposed (see U.S. Pat. No. 4,367,085) that the deposited cylinder be rotated about a vertical axis but that the angle of impingement should be about =40.degree., with an absolute maximum angle for growth of 60.degree.. Further, laminar flow of the soot stream, with Reynolds numbers less than about 100, has been considered to be necessary for deposition control. Published data indicates that the fine glass particle deposition rate drops off at Reynolds numbers above 30-50 and decreases substantially above 80. With these operative limitations, it is not feasible substantially to increase the deposition rate and thereby reduce costs.
Where synchronized axial and radial deposition of core and cladding are used along with an outer sleeving to provide additional cladding, other cost and performance barriers are also encountered. The cladding thickness (t) to core radius (a) ratio has both operative and economic significance. A high t/a ratio can mean lower losses, in accordance with published data, but the initial layers are the most costly to form and the penalty of incurring inordinate costs in the formation of core or cladding is not acceptable. Further, it is necessary to restrict hydroxyl ion content to keep losses low, because OH ions, introduced at interfaces or during processing, are representative of H.sub.2 O content, which is directly proportional to absorption. Nonetheless, most manufacturers using the VAD process build a core and cladding structure of limited t/a ratio of 7 and combine this with a low hydroxyl ion sleeving tube. High costs are involved both in the soot-deposited portion, which is limited in rate and size, and in the incorporation of sleeving, which must be specially prepared. Elimination of these limitations is highly desirable.
It is also known to use a redeposition or "hybrid" process, as discussed in U.S. Pat. No. 4,378,985, for a graded index fiber. In this process the outer cladding is formed by an added soot layer. This approach is subject to the problems of synchronization mentioned above, as between axial and radial deposition.
Consequently, while there has been a constant evolution, in optical waveguide technology, of product properties and performance, subtle and complex interrelationships inhibit further improvements. In addition to those mentioned, signal propagation properties are substantially affected by the shape of the refractive index profile. For single mode propagation, widely used because of its bandwidth potential, the core/cladding ratio needs to be accurately controlled and the interface should provide a predictable "quasi-step" characteristic. The VAD process used with simultaneous soot deposition cannot consistently provide the above characteristics, and consequently there can be large variation of cut-off wavelength and inferior dispersion characteristics. Also, hydroxyl ion migration in the sleeving operation requires use of a large t/a ratio to control hydroxyl content in the fiber. Consequently, the process is expensive and the properties of the resulting fibers are marginal.